Рис.1.  | Basal human Pol I and Pol III transcription complexes.


Рис.2.  | ERK targets that stimulate protein synthesis and therefore cell growth.


Рис.3.  | Cross-talk between p53, ARF and ribosome biosynthesis.


Рис.4.  | The RB pathway has several effects on transcription.


Рис.5.  | Induction of rRNA and tRNA synthesis by mitogens.

Boxes

Box 1 | Eukaryotic RNA polymerases and their products

In eukaryotes, the transcription of nuclear genes is shared by three RNA polymerases (Pols), each of which is essential for viability. Pol I has 14 subunits, whereas Pol II and Pol III have 12 and 17 subunits, respectively. Pol I is dedicated exclusively to transcribing the rRNA genes, of which there are ~400 copies in humans. Pol II transcribes the protein-encoding genes, as well as many genes that encode small nuclear (sn)RNA molecules. Pol III synthesizes various short untranslated RNA molecules, including 5S rRNA, tRNA, 7SL RNA (an essential component of the SIGNAL-RECOGNITION PARTICLE), 7SK RNA (which regulates Pol II transcription), MRP, U6 and H1 RNA molecules (which are required for post-transcriptional processing of rRNA, mRNA and tRNA, respectively). Pol III also transcribes the SHORT INTERSPERSED NUCLEAR ELEMENTS (SINES), including Alu genes, of which there are over a million in humans. Despite having the smallest number of templates, it is thought that Pol I can contribute up to ~70% of all nuclear transcription in actively growing cells, with Pol II and Pol III providing ~20% and ~10%, respectively.

Box 2 | The case for UBF and TFIIIB as bona fide RB targets

More than 70 transcription factors have been reported to bind retinoblastoma protein (RB)¹⁰⁰. Although this sounds unlikely, RB is hundreds of times more abundant than most transcription factors and therefore has the potential to interact with many partners. Furthermore, many of the interactions might be restricted to particular times or cell types. In principle, RB could indeed be involved in regulating this diversity of targets, thereby functioning as a master controller that integrates several activities. However, not all the reported RB-binding partners have strong credentials as bona fide RB targets; for example, some interactions have only been observed in vitro or under conditions of overexpression¹⁰⁰. By contrast, a wealth of biochemical and genetic data establish that transcription by RNA polymerase (Pol) I and Pol III is regulated by RB through its interactions with upstream binding factor (UBF) and the general Pol-III-specific transcription factor TFIIIB, respectively. In both cases, regulation has been confirmed by four independent laboratories^{110-112,114,115,122,145}. The evidence can be summarized as follows:

• Pol III transcription is elevated in primary fibroblasts from RB-knockout mice^53,111,120. The effect can be reproduced in vitro using extracts of these cells¹¹³. Pol I transcription is normal in such cells, but is elevated in RB^-/- p130^-/- double-knockout fibroblasts¹²².
• Nucleolar localization of RB is observed in many cell types^{110,119,146,147}.
• Chromatin immunoprecipitation revealed the presence of endogenous RB at chromosomal U6 snRNA genes in several types of living cell¹¹⁶.
• Overexpression of RB represses transcription by Pol I and Pol III in transfected cells^{54,110-112,115,117,119,130}. Repression is abolished if the transfected RB carries point mutations in its pocket domain^{54,111,112,130}.
• Recombinant RB inhibits transcription by Pol I and Pol III in vitro^{110-114,116,117,122,139,145}. This repression can be prevented by adenoviral E1A¹¹¹, simian virus 40 (SV40) large T antigen¹³⁹ or by point mutations in the RB pocket^110-112.
• RB-mediated repression of Pol I or Pol III transcription can be reversed by overexpression of UBF or TFIIIB, respectively^{110,112-114,145}. This implicates UBF and TFIIIB as targets of RB.
• UBF and TFIIIB bind in GST PULL-DOWN ASSAYS to wild-type, but not mutant, forms of recombinant RB^{110,112-116,118,121,122}.
• Endogenous RB co-immunoprecipitates with endogenous UBF and TFIIIB^{74,113,118,120-122,139}. This interaction is compromised by viral E7 (Ref. 110) or large T antigens of SV40 and polyomavirus^74,139 and when RB is inactivated by phosphorylation^11,120.
• Primary fibroblasts from RB-knockout mice have a specific elevation of TFIIIB activity¹¹³.

Links DATABASES OMIM: Burkitt lymphoma | Li–Fraumeni syndrome Swiss-Prot: ARF | CBP | eIF4E | HDM2 | L11 | MAD1 | Myc | nucleophosmin | p53 | p107 | p130 | RB | SL1 | TBP | TFIIIB | TIF1A | UBF FURTHER INFORMATION Robert White's laboratory